are you an ABAPer? A functional consultant would just use the Envrionment menu in VA03 to look at the changes in a sales order.
I think SAP does not randomly pick any unit, SAP has always tried to be logical and takes the units from preceding documents or master data.
Lets start with the material master.
Is the unit listed in the alternative units?
Is there a an order unit maintained in the purchasing view?
Is this order unit identical to the unit in your purchase order?
Is the indicator for variable order unit set, which number?
Check the purchasing info record for a order unit.
Is the order unit here different to the base unit?
Is it different to your unit from the sales order?
In case your purchase order is made in reference to a contract too, is there eventually a different order unit in the contract?
Can the case be repeated for the same material with the same result?